1. Field of Invention
The present invention relates to a pulse modifier, a lithographic apparatus and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus typically comprises large expensive optical elements that are difficult to fabricate. Typically, an excimer laser is used to supply the lithographic apparatus with a radiation beam in the form of pulses. The optical elements are subject to degradation resulting from billions of these high intensity ultraviolet pulses. Optical damage is known to increase with increasing intensity (i.e., light power (energy/time) per cm2 or mJ/ns/cm2) of the pulses from the laser. The typical pulse length from these lasers is about 20 ns, so a 5 mJ laser pulse would have a pulse power intensity of about 0.25 mJ/ns (0.25 MW). Increasing the pulse energy to 10 mJ without changing the pulse duration would result in a doubling of the power of the pulses to about 0.5 mJ/ns, which could significantly shorten the usable lifetime of the optical elements.
Furthermore, in order to accurately reproduce the pattern of patterning device onto a target portion of a substrate, the radiation beam produced by the laser should be well defined, having substantially constant and predictable properties. The radiation beam should be substantially symmetrical in shape and have a substantially uniform intensity distribution. In practice, no perfectly symmetrical and/or uniform radiation beam is obtainable. For instance, the Cymer XLA-165 laser is known to produce a radiation beam having a constant contour (due to a rectangular diaphragm inside the laser), but have a fluctuating and asymmetric intensity distribution of the radiation beam cross-section. This results in unstable measurements of radiation beam positioning and pointing. High-power lasers in general do not have a constant and symmetric intensity profile—the profile changes in time, e.g., due to laser refills or gas heating or gas burn-up.
A beam modifying configuration has been proposed in U.S. patent application publication no. 2007-0090278 for use with a lithographic apparatus. In that application, the problem of optical damage may be avoided by increasing substantially the pulse length of the pulses of the radiation beam. Furthermore, the problem of inaccurate transfer of the pattern of the patterning device onto a target portion on the substrate due to an asymmetric intensity distribution of the radiation beam cross-section may be reduced by combining delayed copies of the original beam intensity distribution with delayed mirrored copies of the original beam intensity distribution into an output radiation beam (in which mirrored is meant as being mirrored simultaneously in the horizontal as well as in the vertical plane). In this way, an asymmetric intensity distribution of the radiation beam cross-section may be reduced to a certain extent.
A possible disadvantage of a beam modifying configuration as proposed in U.S. patent application publication no. 2007-0090278 is that the asymmetric intensity distribution of the radiation beam cross-section may not be sufficiently reduced. Additional or alternative disadvantages may be a high sensitivity of the outcoming radiation beam quality to the incoming beam divergence and a high sensitivity to alignment of the optical components to each other and to the incoming radiation beam.
It is desirable, for example, to provide a pulse modifier that reduces the changes to the characteristics of the incoming radiation beam.